In a conventional heat exchanger, a core portion is composed by alternately laminating a large number of tubes and corrugated fins on each other. End portions of tubes in the longitudinal direction are inserted and joined to insertion holes formed in a core plate portion attached to a tank. In order to reinforce the core portion, side plates are arranged at both end portions of the core portion in the direction of tube lamination. Concerning this technique, refer to the official gazette of Japanese Utility Model Registration No. 3059971 and the official gazette of JP-A-11-142090.
In this connection, in the case where this heat exchanger is applied to a radiator for vehicle use, engine cooling water does not flow in the side plates but flows in the tubes. The side plates are joined to corrugated fins and cooled by a blast of cooling wind. Therefore, a difference in temperature is generated between the tubes and the side plates. Therefore, a difference in thermal expansion is caused between the tubes and the side plates.
Further, in the case where a volume of cooling wind fluctuates in each part of the core portion, differences in temperature are caused among a large number of tubes according to the positions at which the tubes are arranged. Accordingly, differences in thermal expansion are caused among the tubes.
As a portion for absorbing differences in thermal expansion between the tubes and the side plates is provided in the heat exchanger shown in the above Patent Document, the stress generated by the differences in thermal expansion between the tubes and the side plates can be reduced.
However, it is impossible to absorb all the differences in thermal expansion among the tubes. Therefore, stress caused by differences in thermal expansion among the tubes is generated. Accordingly, when it is attempted to reduce the wall thickness of the tubes as compared with the present wall thickness, there is a possibility that the tubes are broken at the join between the tubes and the core plates by the stress generated by the difference in thermal expansion. Accordingly, there is a possibility that the life of the tubes is deteriorated. For the above reasons, it is impossible to further reduce the wall thickness of the tubes.